I am always 
at the people who say that you should just ask to check his phone and the reaction will tell you everything. Essentially if your partner confronted you and said they wanted to check your phone, would you really just hand it over? Or would you perhaps be outraged/hurt that your partner had so little trust in you that they felt that they had no option but to demand to see your phone?
I can tell you from personal experience that that kind of demand becomes a self-fulfilling prophecy. My eXH used to just pick up my phone and go through it whenever he fancied. There was nothing on it but he felt he had the right to do so even though it was a one way street iyswim - I didn’t have the same access to his phone.
I had nothing to hide but the mere assumption that I was hiding something led me to change my passcode and my passwords to social media after I discovered that he was snooping on those as well and had set them up on his own phone to see what i was doing all of the time.
He put keyloggers on my computer, bugged the house, checked up on me using find my iPhone and then tried to gaslight me by telling me that I’d been seen out with aa man (I was with one actually, a male friend and he knew I was out with him,)
I changed all my passwords etc not because I had anything to hide, but because he clearly didn’t trust me and thought that he had the right to snoop as and when until he found proof of something.
He didn’t find anything on my phone and social media which is why he became more and more desperate. And that’s the danger. If you find nothing on your partner’s phone will you be satisfied? Or will you need to take the next step? Bug the car? Hire a private investigator? Where does it stop?
So no, the talk that someone would just hand their phone over purely because their partner asks for it is bollocks.
If the trust has gone, then the relationship is over.